2,519 research outputs found
Charge-exchange mechanisms at the threshold for inelasticity in Ne+ collisions with surfaces
We present a study on scattering of 100â1400 eV Ne+ ions off Mg, Al, Si, and P surfaces. Exit energy distributions and yields of single-scattered Ne+ and Ne2+ were separately measured to investigate charge exchange mechanisms occurring at the onset of inelastic losses in binary hard collision events. At low incident energies, collisions appear elastic and projectile ion survival is dominated by nonlocal Auger-type neutralization involving the target valence band. However, once a critical Rmin (distance of closest approach) is reached, three phenomena occur simultaneously: Ne2+ generation, reversal of the Ne+ yield trend, and inelastic losses in Ne+ and Ne2+. Rmin values for the Ne2+ turn-on agree very well with the L-shell overlap distances of the colliding partners, suggesting that electron transfer involving the highly promoted 4fsigma molecular orbital (correlated to the Ne 2p) at close internuclear distance (~0.5 Ă
) is responsible. For the Ne+ yield, a clear transition from nonlocal neutralization to Rmin-dependent collision induced neutralization was observed. Binary collision inelasticities (Qbin) were evaluated for Ne+ and Ne2+ off Al and Si by taking into account electron straggling. Saturation-like behavior at RminNe** (2p43s2, 41â45 eV) and Ne+-->Ne+** (2p33s2/3s3p, 69â72 eV), followed by autoionization as the projectile leaves the surface region to give Ne+ and Ne2+. In contrast, Qbin values for Ne2+ at the +2 turn-on were seen much lower (35â40 eV off Al, 55â60 eV off Si) than that required for double promotionâeliminating the possibility that Ne2+ is only generated in double excitation of surviving Ne+. Thus single-electron excitation appears to be more important in the threshold region compared to the two-electron events seen at higher collision energies. In addition, the Ne+[Single Bond]P system shows striking similarities with the other target cases from the perspective of a well-defined Ne2+ turn-on, continually increasing Ne2+ yield with impact energy, and inelasticity values which point to the same 4fsigma excitation pathway. The decreasing Rmin requirement for higher target Z in terms of Ne2+ production has been confirmed for the Mg through P series, where hard collision excitation is governed by L-shell orbital overlaps
Evidence of Simultaneous Double-Electron Promotion in F+ Collisions with Surfaces
A high-flux beam of mass-filtered F+ at low energy (100â1300 eV) was scattered off Al and Si surfaces to study core-level excitations of F0 and F+. Elastic scattering behavior for F+ was observed at energies 450 (700) eV off Al (Si) produces F2+âbehavior which is remarkably similar to Ne+ off the same surfaces. Inelasticities measured for single collision events agree well with the energy deficits required to form (doubly excited) F** and F+** states from F0 and F+, respectively; these excited species most likely decay to inelastic F+ and F2+ via autoionization
Reactions of cyclomanganated complexes with carbon disulfide: routes to Ρ²-aryldithiocarboxylate-Mn(CO)â complexes and to the trithiocarbonate complex (Îźâ-CSâ)âMnâ(CO)ââ
Reaction of cyclomanganated aryl ketones with CSâ proceeds with insertion into the MnâC bond to give Ρ²-dithiocarboxylatoâMn(CO)â compounds. With other cyclomanganated substrates such as that from PhâP=S and also with Mnâ(CO)ââ, CSâ gives (Îźâ-CSâ)âMnâ(CO)ââ with bridging trithiocarbonate ligands
Manganese carbonyl-mediated reactions of azabutadienes with phenylacetylene, methyl acrylate and other unsaturated molecules
Reaction of PhCHâMn(CO)â
with l,4-di-aryl-1-aza-1,3-butadienes gave substituted pyrrolinonyl rings which were Ρâ´-coordinated to a Mn(CO)â group. These are formed by intramolecular CO insertion into a (non-isolated) cyclomanganated intermediate, followed by cyclisation. Other unsaturated reagents (PhCâĄCH, CH2=CHCOOMe, PhNCO) gave products arising from insertion of these, including a structurally characterised tri-aryl-Ρâľ-azacyclohexadienyl-Mn(CO)â complex from the reaction with the alkyne.
PhCHâMn(CO)â
reacts with l,4-di-aryl-1-aza-1,3-butadienes in the presence of unsaturated substrates to give products based on a cyclomanganated intermediate
A phylogenetic analysis of revolution and afterlife beliefs
Beliefs about the fate of humanity and the soul after death may structure behaviours of religious groups. Here we test theories from religious studies: that belief in an imminent apocalypse co-evolved with and facilitated revolutionary violence, whereas belief in reincarnation caused people to acquiesce to existing social orders and withdraw from political activism. We test these hypotheses by building a cultural phylogeny of historical Islamic sects and schools from the seventh to twentieth centuries and use phylogenetic comparative methods to show that these two types of belief display distinct relationships with intergroup violence. There is substantial evidence that apocalyptic beliefs co-evolved with revolutionary violence, whereas reincarnation beliefs were evolutionarily stable in peaceful groups. In both cases, violence precedes the emergence of beliefs, which suggests that conditions that generate revolutionary violence changed beliefs rather than beliefs generating violence. We also found that apocalyptic beliefs are associated with accelerated group extinction, although causal relationships cannot be determined
Electron-acoustic plasma waves: oblique modulation and envelope solitons
Theoretical and numerical studies are presented of the amplitude modulation
of electron-acoustic waves (EAWs) propagating in space plasmas whose
constituents are inertial cold electrons, Boltzmann distributed hot electrons
and stationary ions. Perturbations oblique to the carrier EAW propagation
direction have been considered. The stability analysis, based on a nonlinear
Schroedinger equation (NLSE), reveals that the EAW may become unstable; the
stability criteria depend on the angle between the modulation and
propagation directions. Different types of localized EA excitations are shown
to exist.Comment: 10 pages, 5 figures; to appear in Phys. Rev.
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